The present invention relates to a surface acoustic wave filter and, more particularly, to a weighting transducer for a surface acoustic wave filter.
In general, an SAW (Surface Acoustic Wave) filter is an element used for obtaining filter characteristics as follows. An IDT (interdigital transducer) which is obtained by properly weighting a piezoelectric substrate surface is arranged, and an SAW is transmitted and received by the IDT. As a characteristic feature of the SAW filter, amplitude and phase characteristics can be arbitrarily and independently designed. As a weighting transducer for a conventional surface acoustic wave filter, an apodized transducer and a withdrawal transducer are used. That is, as shown in FIGS. 3A and 3B, in the apodized transducer, overlap widths W obtained by overlapping transducer fingers 21 of a bus electrode 23 and transducer fingers 22 of the bus electrode 24 are locally changed in proportion to a weighting function. In the withdrawal transducer, as shown in FIGS. 4A and 4B, an overlap width 31 is constant, and the density of transducer fingers 32 having an overlap width is proportional to a weighting function.
Each of the above conventional weighting transducers has a drawback, and they are properly used depending on applications. That is, in the apodized method shown in FIGS. 3A and 3B, although a weighting function can be faithfully expressed by the overlap width W, the filter characteristics of a portion having a small weighting coefficient, i.e., a portion having a narrow overlap width, are easily degraded by an error caused by a diffraction effect, and an energy distribution 25 excited from a transducer is not uniform due to the distribution of weighting functions and causes a weighting loss. In addition, in the withdrawal method shown in FIGS. 4A and 4balthough an energy distribution 35 excited from a transducer is uniform, since weighting functions are expressed by a change in density caused by the presence/absence of overlap widths, a quantization error is larger than that of the apodized method, and desired characteristics are not easily obtained.